Vertebrates and the frog’s life cycle

Vertebrates (phylum Chordata) fall into five classes. Fish live in water, with a streamlined body covered in slime; they breathe with gills; the heart has 2 chambers and there is one circulation loop; they are cold-blooded. Amphibians (frog, salamander, newt) live near water and breathe through skin and lungs; the heart has 3 chambers and there are 2 circulation loops; they lay eggs in water, so cannot go far from it. Reptiles (lizard, snake, crocodile) have a body covered in horny scales and breathe with lungs; the heart usually has 3 chambers (4 in crocodiles); they lay hard-shelled eggs on land and are cold-blooded. Birds have bodies covered with feathers and forelimbs turned into wings; air sacs connected to the lungs; a 4-chambered heart and warm blood (constant body temperature); they sit on their eggs. Mammals have hair-covered bodies, feed their young with milk, breathe with lungs, and have a 4-chambered heart and warm blood. “Cold-blooded” means the body temperature depends on the surroundings: when it warms up the animal moves faster, when it cools it slows down. All animals are adapted to their habitat.